1. Field of the Invention
The present invention relates to power converter circuits. More particularly, the invention relates to a multi-phase power converter for use in low duty cycle applications having a circuit that clamps output current overshoot in response to a step down in load.
2. Description of Related Art
Switched mode DC-to-DC power converters are commonly used in the electronics industry to convert an available direct current (DC) level voltage to another DC level voltage. A switched mode converter provides a regulated DC output voltage to a load by selectively storing energy in an output inductor coupled to the load by switching the flow of current into the output inductor. A synchronous buck converter is a particular type of switched mode converter that uses two power switches, typically MOSFET transistors, to control the flow of current in the output inductor. A high-side switch selectively couples the inductor to a first power supply voltage while a low-side switch selectively couples the inductor to a second power supply voltage, such as ground. A pulse width modulation (PWM) control circuit is used to control the gating of the high-side and low-side switches in an alternating manner. Synchronous buck converters generally offer high efficiency and high power density, particularly when MOSFET devices are used due to their relatively low on-resistance. Therefore, synchronous buck converters are advantageous for use in providing power to electronic systems, such as microprocessors that require a control voltage (VCC) of 1 to 1.5 volts with current ranging from 40 to 60 amps.
For certain applications having especially demanding current load requirements, it is known to combine plural synchronous buck converters together in multi-phase configurations operated in an interleaf mode. The output inductors of each of the multiple channels are connected together to provide a single output voltage. Multi-phase configurations are advantageous in that they provide an increase in the frequency of ripple across the output voltage above the switching frequency of the individual channels, thereby enabling the use of smaller output capacitors to reduce the ripple. Also, by spreading the output current among the multiple channels, the stress on individual components of the power converter is reduced.
While it is desired that the power converter deliver a constant output current and voltage to a load, in practice, the output current and voltage fluctuates in response to changing load conditions. The power converter adapts to these changing load conditions by regulating the output current and voltage in response to feedback signals. When there is a step increase in load, there is a corresponding drop in the output current and/or voltage. The power converter adjusts for this changed load condition by increasing the duty cycle applied to the power switches to thereby increase the power delivered to the load. Conversely, when there is a step decrease in load, there is a corresponding rise in the output current and/or voltage that is accommodated by decreasing the duty cycle applied to the power switches to reduce the power delivered to the load. As a result, the change in the output current and/or voltage remains within an allowable limit, and the output current and/or voltage quickly returns to the desired level.
For certain DC-to-DC conversion applications requiring a relatively large reduction of the input voltage (e.g., from 24 volts to 1 volts), it is known to operate a power converter using a relatively low duty cycle (e.g., less than 10%). Such operation allows good response to a step increase in load. Since the duty cycle is already relatively low, there is ample margin to increase the duty cycle to satisfy the load current demand. But, when there is a step decrease in load, there is little margin to further decrease the already low duty cycle in order to reduce the current delivered to the output inductor. Furthermore, since the voltage across the output inductors is so low (e.g., 1 volt), the rate of change of current through the inductor is also very low. The output inductors are therefore not able to quickly respond to the changing load condition and the excess current flows into the load. If the output current rises above the desired level (referred to as current overshoot), there can be significant damage to the load.
Accordingly, it would be desirable to provide an improved way to control the output current of a low duty cycle power converter to avoid current overshoot caused by a step down in load. More particularly, it would be desirable to provide an improved way to control the output current of a multiple-phase, low duty cycle power converter to avoid current overshoot caused by a step down in load.
The present invention overcomes these drawbacks of the prior art by providing a way to control the output current of a low duty cycle power converter to avoid current overshoot caused by a step down in load.
In an embodiment of the invention, a power converter comprises at least one output inductor having an input terminal and an output terminal. The output terminal provides an output voltage therefrom. At least one switch circuit is connected to the input terminal to alternately connect the input terminal to a voltage source and to ground. A ripple control circuit is coupled to the output terminal and is adapted to clamp output current of the output inductor to an allowable maximum level in response to an output current overshoot condition produced by a step down in load coupled to the output terminals. The ripple control circuit further comprises an inductor conducting the output current to ground upon the output current overshoot condition.
More particularly, the ripple control circuit further comprises a capacitor connected in series with the inductor. A high-side switch is coupled to the at least one voltage source and a low-side switch is coupled to ground. The inductor and the capacitor are coupled to a node defined between the high-side switch and the low-side switch. The capacitor is charged by activation of the high-side switch. The capacitor is charged to a voltage substantially higher than the output voltage. The low-side switch is activated upon the output current overshoot condition. The inductor has an inductance substantially lower than the output inductor.
In another embodiment of the invention, the ripple control circuit further comprises a current sensor coupled in parallel with the inductor. The current sensor further comprises a capacitor and resistor coupled in series, and a differential amplifier adapted to measure a voltage across the capacitor. The measured voltage corresponds to the current through the inductor. By arranging the series coupled inductor and capacitor, the measured voltage can be referenced to the output voltage.
A more complete understanding of the method and apparatus for controlling the output current of a low duty cycle power converter to avoid current overshoot caused by a step down in load will be afforded to those skilled in the art, as well as a realization of additional advantages and objects thereof, by a consideration of the following detailed description of the preferred embodiment. Reference will be made to the appended sheets of drawings that will first be described briefly.